Optical surveying instruments, such as a theodolite, may be used, within the visible portion of the electromagnetic spectrum, to measure a distance to a specified point, or one or more distances between specified points in a field of view, such as different points on an object or landscape feature. However, the need exists, in various circumstances, to measure a distance to a specified point, or distances between specified points on one or more objects, where one or more of the specified points cannot be viewed because they are obscured behind an opaque barrier such as a wall or an outer surface of a device.
The use of penetrating radiation and one or more detectors to detect scattered radiation and provide imaging of hidden objects has been described, for example, in U.S. Pat. No. 6,282,260 to Grodzins, issued Aug. 28, 2001, the content of which is incorporated herein by reference. The Grodzins '260 Patent, however, fails to teach how to obtain information with regard to distances between specified points in a hidden object.